This invention relates to optical circulators compatible with optical glass fiber transmission lines (glass-fiber lines) in optical communication systems.
As it is well known, rapid advances have been made in the development of low-attenuation optical waveguides, such as glass-fiber lines, suitable for long-distance communications in the 1.0-1.7 microns wavelength range. In this range, transmission losses in a glass-fiber line have been reported to decrease to 0.3-1 dB/km. With the recent advent of semiconductor lasers and LED's operating in the region of these longer wavelengths, optical communications become more attractive. It is considered that optical communications suffice demands for high quality and large capacity, in addition to light weight and immunity from electrical induction disturbances. However, to realize such optical communication, various optical circuit elements, such as isolators, circulators, filters, couplers, power dividers, and other in addition to, amplifiers, modulators, and demodulators, have to be developed. Most of them certainly will appear in the near future. Especially, an optical circulator plays an indispensable role as a nonreciprocal element in two-way communication. In this communication, one single transmission line is used in common for transmitting two signals in opposite directions, so that signals from a sender are separated from signals for a receiver by use of a circulator. A circulator also acts as an isolator in measuring experiments. Up to date, there has been no optical circulator available with performance characteristics that compares favorably with those of optical glass-fiber lines.
The object of the invention is to provide an optical circulator which is comparible with optical glass-fiber lines, of small size and light weight, and easy to construct.
An optical circulator embodiment of the invention is constructed by combining a magneto-optic (MO) cylinder of a small diameter having a reflective coating at one plane end and three or more glass-fiber lines connected to the other plane end of an MO cylinder, each glass-fiber line being positioned in rotational symmetry to others. Thus, a couplied junction for three or more glass-fiber lines is constructed. Circulator action of this junction takes place under a biasing magnetic field applied parallel to the common axis of the MO cylinder. Detailed explanation of the circulator embodiment of the invention will be made after some physical background is mentioned.
In a circulator embodiment of the invention, an MO cylinder is utilized which is made of magneto-optic crystalline material, such as Al-YIG and Bi-YIG, having large magneto-optic anisotropy under biasing magnetic field. According to the magneto-optic theory, optical wave propagation in the MO material can be described in terms of dielectric tensor permittivity dependent upon the MO crystalline axis of symmetry and the direction of biasing magnetic field. In a sense, the MO anisotropic splitting factor .eta./.epsilon., where .eta. and .epsilon. are non-diagonal and diagonal elements of the tensor permittivity, results in giving rise to nonreciprocal action in the wave propagation in an MO structure, just as does the ferromagnetic anisotropic splitting factor .kappa./.mu., where .kappa. and .mu. are non-diagonal and diagonal elements of the tensor permeability of ferromagnetic material, in the operation of microwave circulators and isolators. Faraday rotation of linearly polarized waves in MO material or ferromagnetic material may be also explained in terms of .eta./.epsilon. or .kappa./.mu..
Actually measured experimentally, the value of .eta./.epsilon. is exceedingly small in comparison with that of .kappa./.mu.. If one wants to rotate linearly polarized optical waves to a sufficient degree of polarization, it is necessary to let the waves pass through as long a distance as many hundreds of wavelengths, since the optical wave length of present interest is in the range of 1.0-1.7 microns. The above distance is still considered to be within several millimeters, but it can be shortened by half when an MO structure has one reflective end for waves to propagate a return path.
To meet the requirement of compatibility with glass-fiber lines, a circulator embodiment of the invention has to implement two mechanisms in the junction. One is to produce on an entry surface of the MO cylinder the desirable wave patterns of various orders so that incident and reflected waves can build up, just like standing wave patterns that azimuthal magnetic fields product in the common region of a microwave stripline Y circulator, and the other to make glass-fiber lines closely coupled with the MO cylinder through the above-mentioned various patterns. According to propagating circular cylindrical wave modes in the MO cylinder having a diameter of more than hundreds of wavelengths, as many propagating wave modes may be excited by incident waves from a coupled glass-fiber line. Appropriate diameter and length of the MO cylinder decreases the possible propagating wave mode numbers through highly precise setting-up of a circulator embodiment of the invention, since the MO cylinder constitutes a high Q resonator. Present day fabrication technology is capable of producing a sufficiently thin MO cylinder with high quality.
Machine-work setting-up and adjustment are generally interrelated. Glass-fiber lines are classed in single mode glass-fiber line (SMGF lines) and multiple mode glass-fiber lines (MMGF lines). If one utilizes SMGF lines or polarization-oriented single mode glass-fiber lines (POSMGF lines) which recently appeared in Japan, prior to MMGF lines, participating mode numbers can be decreased to get stable operation of a circulator, and thereby its broadband operation can be expected. Use of MMGF lines, however, has an advantage in being easy to construct a circulator mainly because this line has a large aperture of coupling.
Setting-up of a junction is none other than the way to make glass-fiber lines couple with the MO cylinder. One method disclosed in U.S. Pat. No. 4,378,951 was that each of that canted off coupled glass-fiber lines was attached to an MO cylinder at its peripheral positions symmetrically located in a common transverse section. That is different from the present way of coupling. The way of direct coupling of glass-fiber lines on the entry plane surface of the MO cylinder which is disclosed in a circulator embodiment of the invention gains an advantage in making use of volume modes among the propagating wave modes of the MO cylinder, thereby diminishing the useless part of the optical wave power propagating near the surface of the MO cylinder.
Adjustment is another matter of concern, dependent chiefly on biasing magnetic field after other factors are all determined. There are several losses of optical wave powers, conversion losses between wave modes of the MO cylinder and coupled glass-fiber lines, radiation losses from the cylindrical and plane surfaces, and dissipation losses in the MO cylinder. Conversion losses can be improved by selecting the aperture of the MO cylinder corresponding to the core of the glass-fiber line and, by covering the entry surface of the MO cylinder with reflective coating except openings for coupled glass-fiber lines. The radiation losses become negligible if the MO cylinder is made to get a high Q resonator, or if the MO cylinder is covered by a reflecting coating. The dissipation losses in the MO cylinder intrinsically depend on crystal growing process, so that crystalline material of high quality is always important.